1. Field of the Invention
The present invention relates to an image forming apparatus having a two-component developing unit and a toner concentration control unit.
2. Description of the Related Art
In the image forming apparatuses such as an electrophotographic printer and a copier in which an electrostatic latent image is visualized employing a two-component developer composed of toner and carrier, when the mixture ratio of toner and carrier or the toner concentration is not adequate within a developing unit for developing the electrostatic latent image with toner, a problem arises that the image density is so thin as to cause a carrier over, or the image density is so high as to increase the fogging. Thus, a toner concentration sensor is provided within the developing unit, and a toner supply apparatus for supplying the toner into the developing unit is controlled to keep the toner concentration constant. For example, as the printing density that is the ratio of image area to printing area is higher, a greater amount of toner is consumed and the toner concentration is lower. In this case, more toner must be supplied according to a decreased amount of toner concentration. A control method therefor involves dividing a deviation amount from a reference toner concentration into several levels, and increasing the supply amount of toner as the sensor value indicates the high level of deviation amount (e.g., refer to JP-A-11-52700).
With this control method, when the image is continuously printed at high printing density, the toner concentration is decreased to a level at which the toner supply amount is above the toner consumption amount, and then begins to increase. However, when the toner concentration is increased, the level is changed, and the toner supply amount is decreased. Again, the toner concentration is decreased and increased repeatedly, and finally stabilized at the level considerably lower than the reference toner concentration.
To solve this problem, in the prior art, when a low level of the toner concentration is continued, the supply amount of toner is increased above the amount of toner usually supplied at that level.